The invention relates to a cable fitting which is filled with a dielectric medium when mounted on a cable, and more particularly to such a fitting having a sealing member of an elastic material. The sealing member adjoins the cable insulation and is clamped in the cable fitting, and prevents the dielectric medium from penetrating into the cable between the cable insulation and the cable sheath. Fittings of this type are particularly useful for power cables.
In this connection cable fitting will be understood to mean cable terminations, cable entrances and joints and such like. After inserting a cable, such fittings are filled under excess pressure with a dielectric medium in a fluid state, such as a highly viscous liquid or gaseous state, for example oil, paraffin wax or insulation gas. In general the electric properties of a cable fitting are detrimentally affected if leaks occur through which the dielectric medium can flow away. If materials of the above-mentioned kind penetrate into the cable, deterioration of the electric properties of the cable may follow. In particular this may, for example, be the case with high voltage power cables whose insulation has a conducting or semi-conducting coating which serves as core shield.
In prior art constructions for sealing synthetic resin insulated high-voltage cables in a fitting, an annular sealing member of a flexible rubber material is usually used which is clamped by means of metal hold-down means against the cable insulation (see, for example, U.S. Pat. No. 3,721,745). Because of variations in the electric load which are accompanied by an increase and a decrease of the temperature of the cable, a synthetic resin insulated cable can expand and shrink. With the prior art construction of the sealing members there is then the risk that the cable insulation is constricted at the place of application of the sealing member. In the long run the seal will then prove to be insufficiently resistant against the dielectric medium, which is generally under an excess pressure, in the fitting. In addition, unwanted field concentrations occur in the insulation material which may give rise to so-called treeing. Besides the radial expansion the temperature cycles due to load variations to which the cable is normally exposed can also effect axial movement of the cable with respect to the fittings. The sealing member must be able to take up this axial movement also. With the construction according to U.S. Pat. No. 3,721,745 this is impossible or possible only to a limited extent because of the excessive friction between the cable insulation and the sealing ring clamped thereto.
A proper seal is also important because core shields are usually, as explained above, not resistant to the usual dielectric media, such as, for example, insulating oil.